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Some of the KLs I've got have had the audio delay circuit in the receiver bypassed. This audio delay is what makes the KL have the nice silent mute action. I believe the delay is bypassed so that the receiver works better in data reception applications rather than voice.
I like the repeater to have the silent mute action, so if the delay has been bypassed, I restore the circuit to original. The process is very simple.
The mod is evident by the resistor soldered to the bottom of the board near the black coax. R168 has been removed to disconnect the delay path, and this resistor completes the connection into the rest of the audio processing circuitry. Its value is 8.2k.
Here you can see R168s location north-west of the blue trimmer.
To reverse the mod, simply remove the resistor from the bottom of the board, and put 1.5kOhm into R168. I used a 1k and a 560r taken from a KL main board wreck.